1. Field of the Invention
The present invention relates to an improvement of an apparatus for Metal Organic Chemical Vapor Deposition (hereinafter abbreviated to MOCVD) for epitaxial growth of compound semiconductor wafers. More particularly, the present invention relates to a MOCVD apparatus which is protected from invasion of undesirable foreign gases.
2. Description of the Related Art
In general, an epitaxial growth method for growing a semiconductor crystal layer on a semiconductor wafer is widely used as a basic technology in semiconductor devices fabrication.
There are three major classifications of the methods for epitaxial growth. These are Chemical Vapor Deposition (abbreviated to CVD), Liquid Phase Epitaxy (abbreviated to LPE) and Physical Vapor Deposition (abbreviated to PVD). Further, among the methods of above-described CVD, there is a method called MOCVD which uses a metal organic gas, such as TMA (Tri-Methyl-Aluminum) or TMG (Tri-Methyl-Gallium). In such MOCVD method, the metal organic gas, as a source of the metal, is pyrolyzed to supply metal atoms for crystal growth.
MOCVD is particularly useful for fabricating a compound semiconductor of III-V or II-VI group, such as AlGaAs (Aluminum-Gallium-Arsenide). MOCVD is effectual for growing a very thin crystal layer, a multi-layer structure, or a polyatomic mixed crystal under precise control of contents and it is useful for mass production of the compound semiconductors. Thus, MOCVD has been becoming more important method in semiconductor industry than before.
However, the epitaxial layer grown in the MOCVD is susceptibly influenced by the contents of the gases in which the epitaxial growth is performed. Particularly, aluminum is most susceptible to be oxidized, being followed by antimony, indium and gallium in order. So, foreign gas containing, for example, oxygen or water vapor oxidize the metals of the epitaxy to worsen the purity of the epitaxial layer. Thus, the foreign gas invasion into the reaction chamber results in deterioration of the characteristics of the fabricated devices. Therefore, special efforts have been made to protect the reaction chamber from the invasion of undesirable foreign gases of the outside.
Among these efforts to prevent foreign gas invasion, following two major methods have been applied to MOCVD apparatus, one is using a nitrogen-gas-filled loading chamber, another is using a load lock chamber, for loading as well as unloading the semiconductor wafers into the reaction chamber.
An apparatus for the above-described first method using a nitrogen gas chamber is schematically illustrated in FIG. 1. The loading process of semiconductor wafer 37 into the reaction chamber 32 is as follows. The door 31c of the nitrogen gas chamber 31 is opened to input a semiconductor wafer from the open air area (referred to hereinafter as outside) into the nitrogen gas chamber 31 and place it on the susceptor 33. At this time, the susceptor 33 is positioned at the place shown by dotted lines in the nitrogen gas chamber 31. The door 31c is closed. An inert gas, for example, a clean purified nitrogen gas is always fed from a gas inlet 31a into the nitrogen gas chamber 31 and drained through a gas outlet 31b. The flow of this nitrogen gas purges undesirable foreign gases, which have been undesirably introduced into the nitrogen chamber 31 from the outside together with the wafers 37 and tools when they were inputted therein, and still staying there. The wafer 37 and its susceptor 33 are pushed into the reaction chamber 32 by a connecting rod 36. The lid 35 to which the connecting rod 36 is connected seals an opening between the reaction chamber 32 and the nitrogen chamber 31 to complete the procedure of loading the wafer. Into the reaction chamber 32, gases for reaction of MOCVD are fed from a gas inlet 32a and drained through a gas outlet 32b, while the wafer 37 together with the susceptor 33 are heated by a heating means, for example a radio frequency coil, (not shown in the figure) provided outside the reaction chamber 32. Thus, the MOCVD is performed. The procedure for unloading the wafer from the reaction chamber 32 is the reverse of above-described loading procedure.
Though foreign gases in the nitrogen gas chamber 31 have been purged to some degree by above-described nitrogen gas flow, there still may remain some foreign gases and may invade the reaction chamber 32, resulting in contamination of epitaxial layer of the wafer 37, i.e. oxidization of the metals therein.
In the second method, the load lock chamber is used as an alternative to the nitrogen gas chamber 31 for foreign gas protection. This load lock chamber is not shown by figure, but is similar to the nitrogen gas chamber 31 and provided in place of it. The load lock chamber containing wafers 37 inputted therein is evacuated to remove the gases therein before the wafer is loaded into the reaction chamber 32. This method is more effective to reduce foreign gas invasion into the reaction chamber 32 than the nitrogen gas chamber method, however, there still remain foreign gases which are adsorbed on the wafers, the tools or the walls of the load lock chamber and may then invade the reaction chamber
If perfect removal of these remaining foreign gases is attempted, the load lock chamber must be evacuated in much higher vacuum or baked up to an elevated temperature, requiring more operation time and more sophisticated apparatus. Thus, quality of the semiconductor product in the prior art is not satisfactory even though the efficiency of the production is satisfactory.
Therefore, there still exists a need for a method providing more perfect protection of the reaction chamber from foreign gas invasion while consistently keeping efficient production.